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<div class="subsection-level-extent" id="Common-Function-Attributes">
<div class="nav-panel">
<p>
Next: <a href="AArch64-Function-Attributes.html" accesskey="n" rel="next">AArch64 Function Attributes</a>, Up: <a href="Function-Attributes.html" accesskey="u" rel="up">Declaring Attributes of Functions</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<h4 class="subsection" id="Common-Function-Attributes-1"><span>6.33.1 Common Function Attributes<a class="copiable-link" href="#Common-Function-Attributes-1"> &para;</a></span></h4>

<p>The following attributes are supported on most targets.
</p>
<dl class="table">
<dt><code class="code">access (<var class="var">access-mode</var>, <var class="var">ref-index</var>)</code></dt>
<dt><code class="code">access (<var class="var">access-mode</var>, <var class="var">ref-index</var>, <var class="var">size-index</var>)</code></dt>
<dd>
<p>The <code class="code">access</code> attribute enables the detection of invalid or unsafe
accesses by functions to which they apply or their callers, as well as
write-only accesses to objects that are never read from.  Such accesses
may be diagnosed by warnings such as <samp class="option">-Wstringop-overflow</samp>,
<samp class="option">-Wuninitialized</samp>, <samp class="option">-Wunused</samp>, and others.
</p>
<p>The <code class="code">access</code> attribute specifies that a function to whose by-reference
arguments the attribute applies accesses the referenced object according to
<var class="var">access-mode</var>.  The <var class="var">access-mode</var> argument is required and must be
one of four names: <code class="code">read_only</code>, <code class="code">read_write</code>, <code class="code">write_only</code>,
or <code class="code">none</code>.  The remaining two are positional arguments.
</p>
<p>The required <var class="var">ref-index</var> positional argument  denotes a function
argument of pointer (or in C++, reference) type that is subject to
the access.  The same pointer argument can be referenced by at most one
distinct <code class="code">access</code> attribute.
</p>
<p>The optional <var class="var">size-index</var> positional argument denotes a function
argument of integer type that specifies the maximum size of the access.
The size is the number of elements of the type referenced by <var class="var">ref-index</var>,
or the number of bytes when the pointer type is <code class="code">void*</code>.  When no
<var class="var">size-index</var> argument is specified, the pointer argument must be either
null or point to a space that is suitably aligned and large for at least one
object of the referenced type (this implies that a past-the-end pointer is
not a valid argument).  The actual size of the access may be less but it
must not be more.
</p>
<p>The <code class="code">read_only</code> access mode specifies that the pointer to which it
applies is used to read the referenced object but not write to it.  Unless
the argument specifying the size of the access denoted by <var class="var">size-index</var>
is zero, the referenced object must be initialized.  The mode implies
a stronger guarantee than the <code class="code">const</code> qualifier which, when cast away
from a pointer, does not prevent the pointed-to object from being modified.
Examples of the use of the <code class="code">read_only</code> access mode is the argument to
the <code class="code">puts</code> function, or the second and third arguments to
the <code class="code">memcpy</code> function.
</p>
<div class="example smallexample">
<pre class="example-preformatted">__attribute__ ((access (read_only, 1)))
int puts (const char*);

__attribute__ ((access (read_only, 2, 3)))
void* memcpy (void*, const void*, size_t);
</pre></div>

<p>The <code class="code">read_write</code> access mode applies to arguments of pointer types
without the <code class="code">const</code> qualifier.  It specifies that the pointer to which
it applies is used to both read and write the referenced object.  Unless
the argument specifying the size of the access denoted by <var class="var">size-index</var>
is zero, the object referenced by the pointer must be initialized.  An example
of the use of the <code class="code">read_write</code> access mode is the first argument to
the <code class="code">strcat</code> function.
</p>
<div class="example smallexample">
<pre class="example-preformatted">__attribute__ ((access (read_write, 1), access (read_only, 2)))
char* strcat (char*, const char*);
</pre></div>

<p>The <code class="code">write_only</code> access mode applies to arguments of pointer types
without the <code class="code">const</code> qualifier.  It specifies that the pointer to which
it applies is used to write to the referenced object but not read from it.
The object referenced by the pointer need not be initialized.  An example
of the use of the <code class="code">write_only</code> access mode is the first argument to
the <code class="code">strcpy</code> function, or the first two arguments to the <code class="code">fgets</code>
function.
</p>
<div class="example smallexample">
<pre class="example-preformatted">__attribute__ ((access (write_only, 1), access (read_only, 2)))
char* strcpy (char*, const char*);

__attribute__ ((access (write_only, 1, 2), access (read_write, 3)))
int fgets (char*, int, FILE*);
</pre></div>

<p>The access mode <code class="code">none</code> specifies that the pointer to which it applies
is not used to access the referenced object at all.  Unless the pointer is
null the pointed-to object must exist and have at least the size as denoted
by the <var class="var">size-index</var> argument.  When the optional <var class="var">size-index</var>
argument is omitted for an argument of <code class="code">void*</code> type the actual pointer
agument is ignored.  The referenced object need not be initialized.
The mode is intended to be used as a means to help validate the expected
object size, for example in functions that call <code class="code">__builtin_object_size</code>.
See <a class="xref" href="Object-Size-Checking.html">Object Size Checking</a>.
</p>
<p>Note that the <code class="code">access</code> attribute merely specifies how an object
referenced by the pointer argument can be accessed; it does not imply that
an access <strong class="strong">will</strong> happen.  Also, the <code class="code">access</code> attribute does not
imply the attribute <code class="code">nonnull</code>; it may be appropriate to add both attributes
at the declaration of a function that unconditionally manipulates a buffer via
a pointer argument.  See the <code class="code">nonnull</code> attribute for more information and
caveats.
</p>
</dd>
<dt><a id="index-alias-function-attribute"></a><span><code class="code">alias (&quot;<var class="var">target</var>&quot;)</code><a class="copiable-link" href="#index-alias-function-attribute"> &para;</a></span></dt>
<dd><p>The <code class="code">alias</code> attribute causes the declaration to be emitted as an alias
for another symbol, which must have been previously declared with the same
type, and for variables, also the same size and alignment.  Declaring an alias
with a different type than the target is undefined and may be diagnosed.  As
an example, the following declarations:
</p>
<div class="example smallexample">
<pre class="example-preformatted">void __f () { /* <span class="r">Do something.</span> */; }
void f () __attribute__ ((weak, alias (&quot;__f&quot;)));
</pre></div>

<p>define &lsquo;<samp class="samp">f</samp>&rsquo; to be a weak alias for &lsquo;<samp class="samp">__f</samp>&rsquo;.  In C++, the mangled name
for the target must be used.  It is an error if &lsquo;<samp class="samp">__f</samp>&rsquo; is not defined in
the same translation unit.
</p>
<p>This attribute requires assembler and object file support,
and may not be available on all targets.
</p>
</dd>
<dt><a id="index-aligned-function-attribute"></a><span><code class="code">aligned</code><a class="copiable-link" href="#index-aligned-function-attribute"> &para;</a></span></dt>
<dt><code class="code">aligned (<var class="var">alignment</var>)</code></dt>
<dd><p>The <code class="code">aligned</code> attribute specifies a minimum alignment for
the first instruction of the function, measured in bytes.  When specified,
<var class="var">alignment</var> must be an integer constant power of 2.  Specifying no
<var class="var">alignment</var> argument implies the ideal alignment for the target.
The <code class="code">__alignof__</code> operator can be used to determine what that is
(see <a class="pxref" href="Alignment.html">Determining the Alignment of Functions, Types or Variables</a>).  The attribute has no effect when a definition for
the function is not provided in the same translation unit.
</p>
<p>The attribute cannot be used to decrease the alignment of a function
previously declared with a more restrictive alignment; only to increase
it.  Attempts to do otherwise are diagnosed.  Some targets specify
a minimum default alignment for functions that is greater than 1.  On
such targets, specifying a less restrictive alignment is silently ignored.
Using the attribute overrides the effect of the <samp class="option">-falign-functions</samp>
(see <a class="pxref" href="Optimize-Options.html">Options That Control Optimization</a>) option for this function.
</p>
<p>Note that the effectiveness of <code class="code">aligned</code> attributes may be
limited by inherent limitations in the system linker 
and/or object file format.  On some systems, the
linker is only able to arrange for functions to be aligned up to a
certain maximum alignment.  (For some linkers, the maximum supported
alignment may be very very small.)  See your linker documentation for
further information.
</p>
<p>The <code class="code">aligned</code> attribute can also be used for variables and fields
(see <a class="pxref" href="Variable-Attributes.html">Specifying Attributes of Variables</a>.)
</p>
</dd>
<dt><a id="index-alloc_005falign-function-attribute"></a><span><code class="code">alloc_align (<var class="var">position</var>)</code><a class="copiable-link" href="#index-alloc_005falign-function-attribute"> &para;</a></span></dt>
<dd><p>The <code class="code">alloc_align</code> attribute may be applied to a function that
returns a pointer and takes at least one argument of an integer or
enumerated type.
It indicates that the returned pointer is aligned on a boundary given
by the function argument at <var class="var">position</var>.  Meaningful alignments are
powers of 2 greater than one.  GCC uses this information to improve
pointer alignment analysis.
</p>
<p>The function parameter denoting the allocated alignment is specified by
one constant integer argument whose number is the argument of the attribute.
Argument numbering starts at one.
</p>
<p>For instance,
</p>
<div class="example smallexample">
<pre class="example-preformatted">void* my_memalign (size_t, size_t) __attribute__ ((alloc_align (1)));
</pre></div>

<p>declares that <code class="code">my_memalign</code> returns memory with minimum alignment
given by parameter 1.
</p>
</dd>
<dt><a id="index-alloc_005fsize-function-attribute"></a><span><code class="code">alloc_size (<var class="var">position</var>)</code><a class="copiable-link" href="#index-alloc_005fsize-function-attribute"> &para;</a></span></dt>
<dt><code class="code">alloc_size (<var class="var">position-1</var>, <var class="var">position-2</var>)</code></dt>
<dd><p>The <code class="code">alloc_size</code> attribute may be applied to a function that
returns a pointer and takes at least one argument of an integer or
enumerated type.
It indicates that the returned pointer points to memory whose size is
given by the function argument at <var class="var">position-1</var>, or by the product
of the arguments at <var class="var">position-1</var> and <var class="var">position-2</var>.  Meaningful
sizes are positive values less than <code class="code">PTRDIFF_MAX</code>.  GCC uses this
information to improve the results of <code class="code">__builtin_object_size</code>.
</p>
<p>The function parameter(s) denoting the allocated size are specified by
one or two integer arguments supplied to the attribute.  The allocated size
is either the value of the single function argument specified or the product
of the two function arguments specified.  Argument numbering starts at
one for ordinary functions, and at two for C++ non-static member functions.
</p>
<p>For instance,
</p>
<div class="example smallexample">
<pre class="example-preformatted">void* my_calloc (size_t, size_t) __attribute__ ((alloc_size (1, 2)));
void* my_realloc (void*, size_t) __attribute__ ((alloc_size (2)));
</pre></div>

<p>declares that <code class="code">my_calloc</code> returns memory of the size given by
the product of parameter 1 and 2 and that <code class="code">my_realloc</code> returns memory
of the size given by parameter 2.
</p>
</dd>
<dt><a id="index-always_005finline-function-attribute"></a><span><code class="code">always_inline</code><a class="copiable-link" href="#index-always_005finline-function-attribute"> &para;</a></span></dt>
<dd><p>Generally, functions are not inlined unless optimization is specified.
For functions declared inline, this attribute inlines the function
independent of any restrictions that otherwise apply to inlining.
Failure to inline such a function is diagnosed as an error.
Note that if such a function is called indirectly the compiler may
or may not inline it depending on optimization level and a failure
to inline an indirect call may or may not be diagnosed.
</p>
</dd>
<dt><a id="index-artificial-function-attribute"></a><span><code class="code">artificial</code><a class="copiable-link" href="#index-artificial-function-attribute"> &para;</a></span></dt>
<dd><p>This attribute is useful for small inline wrappers that if possible
should appear during debugging as a unit.  Depending on the debug
info format it either means marking the function as artificial
or using the caller location for all instructions within the inlined
body.
</p>
</dd>
<dt><a id="index-assume_005faligned-function-attribute"></a><span><code class="code">assume_aligned (<var class="var">alignment</var>)</code><a class="copiable-link" href="#index-assume_005faligned-function-attribute"> &para;</a></span></dt>
<dt><code class="code">assume_aligned (<var class="var">alignment</var>, <var class="var">offset</var>)</code></dt>
<dd><p>The <code class="code">assume_aligned</code> attribute may be applied to a function that
returns a pointer.  It indicates that the returned pointer is aligned
on a boundary given by <var class="var">alignment</var>.  If the attribute has two
arguments, the second argument is misalignment <var class="var">offset</var>.  Meaningful
values of <var class="var">alignment</var> are powers of 2 greater than one.  Meaningful
values of <var class="var">offset</var> are greater than zero and less than <var class="var">alignment</var>.
</p>
<p>For instance
</p>
<div class="example smallexample">
<pre class="example-preformatted">void* my_alloc1 (size_t) __attribute__((assume_aligned (16)));
void* my_alloc2 (size_t) __attribute__((assume_aligned (32, 8)));
</pre></div>

<p>declares that <code class="code">my_alloc1</code> returns 16-byte aligned pointers and
that <code class="code">my_alloc2</code> returns a pointer whose value modulo 32 is equal
to 8.
</p>
</dd>
<dt><a id="index-cold-function-attribute"></a><span><code class="code">cold</code><a class="copiable-link" href="#index-cold-function-attribute"> &para;</a></span></dt>
<dd><p>The <code class="code">cold</code> attribute on functions is used to inform the compiler that
the function is unlikely to be executed.  The function is optimized for
size rather than speed and on many targets it is placed into a special
subsection of the text section so all cold functions appear close together,
improving code locality of non-cold parts of program.  The paths leading
to calls of cold functions within code are marked as unlikely by the branch
prediction mechanism.  It is thus useful to mark functions used to handle
unlikely conditions, such as <code class="code">perror</code>, as cold to improve optimization
of hot functions that do call marked functions in rare occasions.
</p>
<p>When profile feedback is available, via <samp class="option">-fprofile-use</samp>, cold functions
are automatically detected and this attribute is ignored.
</p>
</dd>
<dt><a class="index-entry-id" id="index-functions-that-have-no-side-effects"></a>
<a id="index-const-function-attribute"></a><span><code class="code">const</code><a class="copiable-link" href="#index-const-function-attribute"> &para;</a></span></dt>
<dd><p>Calls to functions whose return value is not affected by changes to
the observable state of the program and that have no observable effects
on such state other than to return a value may lend themselves to
optimizations such as common subexpression elimination.  Declaring such
functions with the <code class="code">const</code> attribute allows GCC to avoid emitting
some calls in repeated invocations of the function with the same argument
values.
</p>
<p>For example,
</p>
<div class="example smallexample">
<pre class="example-preformatted">int square (int) __attribute__ ((const));
</pre></div>

<p>tells GCC that subsequent calls to function <code class="code">square</code> with the same
argument value can be replaced by the result of the first call regardless
of the statements in between.
</p>
<p>The <code class="code">const</code> attribute prohibits a function from reading objects
that affect its return value between successive invocations.  However,
functions declared with the attribute can safely read objects that do
not change their return value, such as non-volatile constants.
</p>
<p>The <code class="code">const</code> attribute imposes greater restrictions on a function&rsquo;s
definition than the similar <code class="code">pure</code> attribute.  Declaring the same
function with both the <code class="code">const</code> and the <code class="code">pure</code> attribute is
diagnosed.  Because a const function cannot have any observable side
effects it does not make sense for it to return <code class="code">void</code>.  Declaring
such a function is diagnosed.
</p>
<a class="index-entry-id" id="index-pointer-arguments"></a>
<p>Note that a function that has pointer arguments and examines the data
pointed to must <em class="emph">not</em> be declared <code class="code">const</code> if the pointed-to
data might change between successive invocations of the function.  In
general, since a function cannot distinguish data that might change
from data that cannot, const functions should never take pointer or,
in C++, reference arguments. Likewise, a function that calls a non-const
function usually must not be const itself.
</p>
</dd>
<dt><a class="index-entry-id" id="index-destructor-function-attribute"></a>
<a id="index-constructor-function-attribute"></a><span><code class="code">constructor</code><a class="copiable-link" href="#index-constructor-function-attribute"> &para;</a></span></dt>
<dt><code class="code">destructor</code></dt>
<dt><code class="code">constructor (<var class="var">priority</var>)</code></dt>
<dt><code class="code">destructor (<var class="var">priority</var>)</code></dt>
<dd><p>The <code class="code">constructor</code> attribute causes the function to be called
automatically before execution enters <code class="code">main ()</code>.  Similarly, the
<code class="code">destructor</code> attribute causes the function to be called
automatically after <code class="code">main ()</code> completes or <code class="code">exit ()</code> is
called.  Functions with these attributes are useful for
initializing data that is used implicitly during the execution of
the program.
</p>
<p>On some targets the attributes also accept an integer argument to
specify a priority to control the order in which constructor and
destructor functions are run.  A constructor
with a smaller priority number runs before a constructor with a larger
priority number; the opposite relationship holds for destructors.  Note
that priorities 0-100 are reserved.  So, if you have a constructor that
allocates a resource and a destructor that deallocates the same
resource, both functions typically have the same priority.  The
priorities for constructor and destructor functions are the same as
those specified for namespace-scope C++ objects (see <a class="pxref" href="C_002b_002b-Attributes.html">C++-Specific Variable, Function, and Type Attributes</a>).
However, at present, the order in which constructors for C++ objects
with static storage duration and functions decorated with attribute
<code class="code">constructor</code> are invoked is unspecified. In mixed declarations,
attribute <code class="code">init_priority</code> can be used to impose a specific ordering.
</p>
<p>Using the argument forms of the <code class="code">constructor</code> and <code class="code">destructor</code>
attributes on targets where the feature is not supported is rejected with
an error.
</p>
</dd>
<dt><a id="index-copy-function-attribute"></a><span><code class="code">copy</code><a class="copiable-link" href="#index-copy-function-attribute"> &para;</a></span></dt>
<dt><code class="code">copy (<var class="var">function</var>)</code></dt>
<dd><p>The <code class="code">copy</code> attribute applies the set of attributes with which
<var class="var">function</var> has been declared to the declaration of the function
to which the attribute is applied.  The attribute is designed for
libraries that define aliases or function resolvers that are expected
to specify the same set of attributes as their targets.  The <code class="code">copy</code>
attribute can be used with functions, variables, or types.  However,
the kind of symbol to which the attribute is applied (either function
or variable) must match the kind of symbol to which the argument refers.
The <code class="code">copy</code> attribute copies only syntactic and semantic attributes
but not attributes that affect a symbol&rsquo;s linkage or visibility such as
<code class="code">alias</code>, <code class="code">visibility</code>, or <code class="code">weak</code>.  The <code class="code">deprecated</code>
and <code class="code">target_clones</code> attribute are also not copied.
See <a class="xref" href="Common-Type-Attributes.html">Common Type Attributes</a>.
See <a class="xref" href="Common-Variable-Attributes.html">Common Variable Attributes</a>.
</p>
<p>For example, the <var class="var">StrongAlias</var> macro below makes use of the <code class="code">alias</code>
and <code class="code">copy</code> attributes to define an alias named <var class="var">alloc</var> for function
<var class="var">allocate</var> declared with attributes <var class="var">alloc_size</var>, <var class="var">malloc</var>, and
<var class="var">nothrow</var>.  Thanks to the <code class="code">__typeof__</code> operator the alias has
the same type as the target function.  As a result of the <code class="code">copy</code>
attribute the alias also shares the same attributes as the target.
</p>
<div class="example smallexample">
<pre class="example-preformatted">#define StrongAlias(TargetFunc, AliasDecl)  \
  extern __typeof__ (TargetFunc) AliasDecl  \
    __attribute__ ((alias (#TargetFunc), copy (TargetFunc)));

extern __attribute__ ((alloc_size (1), malloc, nothrow))
  void* allocate (size_t);
StrongAlias (allocate, alloc);
</pre></div>

</dd>
<dt><a id="index-deprecated-function-attribute"></a><span><code class="code">deprecated</code><a class="copiable-link" href="#index-deprecated-function-attribute"> &para;</a></span></dt>
<dt><code class="code">deprecated (<var class="var">msg</var>)</code></dt>
<dd><p>The <code class="code">deprecated</code> attribute results in a warning if the function
is used anywhere in the source file.  This is useful when identifying
functions that are expected to be removed in a future version of a
program.  The warning also includes the location of the declaration
of the deprecated function, to enable users to easily find further
information about why the function is deprecated, or what they should
do instead.  Note that the warnings only occurs for uses:
</p>
<div class="example smallexample">
<pre class="example-preformatted">int old_fn () __attribute__ ((deprecated));
int old_fn ();
int (*fn_ptr)() = old_fn;
</pre></div>

<p>results in a warning on line 3 but not line 2.  The optional <var class="var">msg</var>
argument, which must be a string, is printed in the warning if
present.
</p>
<p>The <code class="code">deprecated</code> attribute can also be used for variables and
types (see <a class="pxref" href="Variable-Attributes.html">Specifying Attributes of Variables</a>, see <a class="pxref" href="Type-Attributes.html">Specifying Attributes of Types</a>.)
</p>
<p>The message attached to the attribute is affected by the setting of
the <samp class="option">-fmessage-length</samp> option.
</p>
</dd>
<dt><a id="index-unavailable-function-attribute"></a><span><code class="code">unavailable</code><a class="copiable-link" href="#index-unavailable-function-attribute"> &para;</a></span></dt>
<dt><code class="code">unavailable (<var class="var">msg</var>)</code></dt>
<dd><p>The <code class="code">unavailable</code> attribute results in an error if the function
is used anywhere in the source file.  This is useful when identifying
functions that have been removed from a particular variation of an
interface.  Other than emitting an error rather than a warning, the
<code class="code">unavailable</code> attribute behaves in the same manner as
<code class="code">deprecated</code>.
</p>
<p>The <code class="code">unavailable</code> attribute can also be used for variables and
types (see <a class="pxref" href="Variable-Attributes.html">Specifying Attributes of Variables</a>, see <a class="pxref" href="Type-Attributes.html">Specifying Attributes of Types</a>.)
</p>
</dd>
<dt><a class="index-entry-id" id="index-warning-function-attribute"></a>
<a id="index-error-function-attribute"></a><span><code class="code">error (&quot;<var class="var">message</var>&quot;)</code><a class="copiable-link" href="#index-error-function-attribute"> &para;</a></span></dt>
<dt><code class="code">warning (&quot;<var class="var">message</var>&quot;)</code></dt>
<dd><p>If the <code class="code">error</code> or <code class="code">warning</code> attribute 
is used on a function declaration and a call to such a function
is not eliminated through dead code elimination or other optimizations, 
an error or warning (respectively) that includes <var class="var">message</var> is diagnosed.  
This is useful
for compile-time checking, especially together with <code class="code">__builtin_constant_p</code>
and inline functions where checking the inline function arguments is not
possible through <code class="code">extern char [(condition) ? 1 : -1];</code> tricks.
</p>
<p>While it is possible to leave the function undefined and thus invoke
a link failure (to define the function with
a message in <code class="code">.gnu.warning*</code> section),
when using these attributes the problem is diagnosed
earlier and with exact location of the call even in presence of inline
functions or when not emitting debugging information.
</p>
</dd>
<dt><a id="index-externally_005fvisible-function-attribute"></a><span><code class="code">externally_visible</code><a class="copiable-link" href="#index-externally_005fvisible-function-attribute"> &para;</a></span></dt>
<dd><p>This attribute, attached to a global variable or function, nullifies
the effect of the <samp class="option">-fwhole-program</samp> command-line option, so the
object remains visible outside the current compilation unit.
</p>
<p>If <samp class="option">-fwhole-program</samp> is used together with <samp class="option">-flto</samp> and 
<code class="command">gold</code> is used as the linker plugin, 
<code class="code">externally_visible</code> attributes are automatically added to functions 
(not variable yet due to a current <code class="command">gold</code> issue) 
that are accessed outside of LTO objects according to resolution file
produced by <code class="command">gold</code>.
For other linkers that cannot generate resolution file,
explicit <code class="code">externally_visible</code> attributes are still necessary.
</p>
</dd>
<dt><a id="index-fd_005farg-function-attribute"></a><span><code class="code">fd_arg</code><a class="copiable-link" href="#index-fd_005farg-function-attribute"> &para;</a></span></dt>
<dt><code class="code">fd_arg (<var class="var">N</var>)</code></dt>
<dd><p>The <code class="code">fd_arg</code> attribute may be applied to a function that takes an open
file descriptor at referenced argument <var class="var">N</var>.
</p>
<p>It indicates that the passed filedescriptor must not have been closed.
Therefore, when the analyzer is enabled with <samp class="option">-fanalyzer</samp>, the
analyzer may emit a <samp class="option">-Wanalyzer-fd-use-after-close</samp> diagnostic
if it detects a code path in which a function with this attribute is
called with a closed file descriptor.
</p>
<p>The attribute also indicates that the file descriptor must have been checked for
validity before usage. Therefore, analyzer may emit
<samp class="option">-Wanalyzer-fd-use-without-check</samp> diagnostic if it detects a code path in
which a function with this attribute is called with a file descriptor that has
not been checked for validity.
</p>
</dd>
<dt><a id="index-fd_005farg_005fread-function-attribute"></a><span><code class="code">fd_arg_read</code><a class="copiable-link" href="#index-fd_005farg_005fread-function-attribute"> &para;</a></span></dt>
<dt><code class="code">fd_arg_read (<var class="var">N</var>)</code></dt>
<dd><p>The <code class="code">fd_arg_read</code> is identical to <code class="code">fd_arg</code>, but with the additional
requirement that it might read from the file descriptor, and thus, the file
descriptor must not have been opened as write-only.
</p>
<p>The analyzer may emit a <samp class="option">-Wanalyzer-access-mode-mismatch</samp>
diagnostic if it detects a code path in which a function with this
attribute is called on a file descriptor opened with <code class="code">O_WRONLY</code>.
</p>
</dd>
<dt><a id="index-fd_005farg_005fwrite-function-attribute"></a><span><code class="code">fd_arg_write</code><a class="copiable-link" href="#index-fd_005farg_005fwrite-function-attribute"> &para;</a></span></dt>
<dt><code class="code">fd_arg_write (<var class="var">N</var>)</code></dt>
<dd><p>The <code class="code">fd_arg_write</code> is identical to <code class="code">fd_arg_read</code> except that the
analyzer may emit a <samp class="option">-Wanalyzer-access-mode-mismatch</samp> diagnostic if
it detects a code path in which a function with this attribute is called on a
file descriptor opened with <code class="code">O_RDONLY</code>.
</p>
</dd>
<dt><a id="index-flatten-function-attribute"></a><span><code class="code">flatten</code><a class="copiable-link" href="#index-flatten-function-attribute"> &para;</a></span></dt>
<dd><p>Generally, inlining into a function is limited.  For a function marked with
this attribute, every call inside this function is inlined, if possible.
Functions declared with attribute <code class="code">noinline</code> and similar are not
inlined.  Whether the function itself is considered for inlining depends
on its size and the current inlining parameters.
</p>
</dd>
<dt><a class="index-entry-id" id="index-functions-with-printf_002c-scanf_002c-strftime-or-strfmon-style-arguments"></a>
<a class="index-entry-id" id="index-Wformat-3"></a>
<a id="index-format-function-attribute"></a><span><code class="code">format (<var class="var">archetype</var>, <var class="var">string-index</var>, <var class="var">first-to-check</var>)</code><a class="copiable-link" href="#index-format-function-attribute"> &para;</a></span></dt>
<dd><p>The <code class="code">format</code> attribute specifies that a function takes <code class="code">printf</code>,
<code class="code">scanf</code>, <code class="code">strftime</code> or <code class="code">strfmon</code> style arguments that
should be type-checked against a format string.  For example, the
declaration:
</p>
<div class="example smallexample">
<pre class="example-preformatted">extern int
my_printf (void *my_object, const char *my_format, ...)
      __attribute__ ((format (printf, 2, 3)));
</pre></div>

<p>causes the compiler to check the arguments in calls to <code class="code">my_printf</code>
for consistency with the <code class="code">printf</code> style format string argument
<code class="code">my_format</code>.
</p>
<p>The parameter <var class="var">archetype</var> determines how the format string is
interpreted, and should be <code class="code">printf</code>, <code class="code">scanf</code>, <code class="code">strftime</code>,
<code class="code">gnu_printf</code>, <code class="code">gnu_scanf</code>, <code class="code">gnu_strftime</code> or
<code class="code">strfmon</code>.  (You can also use <code class="code">__printf__</code>,
<code class="code">__scanf__</code>, <code class="code">__strftime__</code> or <code class="code">__strfmon__</code>.)  On
MinGW targets, <code class="code">ms_printf</code>, <code class="code">ms_scanf</code>, and
<code class="code">ms_strftime</code> are also present.
<var class="var">archetype</var> values such as <code class="code">printf</code> refer to the formats accepted
by the system&rsquo;s C runtime library,
while values prefixed with &lsquo;<samp class="samp">gnu_</samp>&rsquo; always refer
to the formats accepted by the GNU C Library.  On Microsoft Windows
targets, values prefixed with &lsquo;<samp class="samp">ms_</samp>&rsquo; refer to the formats accepted by the
<samp class="file">msvcrt.dll</samp> library.
The parameter <var class="var">string-index</var>
specifies which argument is the format string argument (starting
from 1), while <var class="var">first-to-check</var> is the number of the first
argument to check against the format string.  For functions
where the arguments are not available to be checked (such as
<code class="code">vprintf</code>), specify the third parameter as zero.  In this case the
compiler only checks the format string for consistency.  For
<code class="code">strftime</code> formats, the third parameter is required to be zero.
Since non-static C++ methods have an implicit <code class="code">this</code> argument, the
arguments of such methods should be counted from two, not one, when
giving values for <var class="var">string-index</var> and <var class="var">first-to-check</var>.
</p>
<p>In the example above, the format string (<code class="code">my_format</code>) is the second
argument of the function <code class="code">my_print</code>, and the arguments to check
start with the third argument, so the correct parameters for the format
attribute are 2 and 3.
</p>
<a class="index-entry-id" id="index-ffreestanding-3"></a>
<a class="index-entry-id" id="index-fno_002dbuiltin-2"></a>
<p>The <code class="code">format</code> attribute allows you to identify your own functions
that take format strings as arguments, so that GCC can check the
calls to these functions for errors.  The compiler always (unless
<samp class="option">-ffreestanding</samp> or <samp class="option">-fno-builtin</samp> is used) checks formats
for the standard library functions <code class="code">printf</code>, <code class="code">fprintf</code>,
<code class="code">sprintf</code>, <code class="code">scanf</code>, <code class="code">fscanf</code>, <code class="code">sscanf</code>, <code class="code">strftime</code>,
<code class="code">vprintf</code>, <code class="code">vfprintf</code> and <code class="code">vsprintf</code> whenever such
warnings are requested (using <samp class="option">-Wformat</samp>), so there is no need to
modify the header file <samp class="file">stdio.h</samp>.  In C99 mode, the functions
<code class="code">snprintf</code>, <code class="code">vsnprintf</code>, <code class="code">vscanf</code>, <code class="code">vfscanf</code> and
<code class="code">vsscanf</code> are also checked.  Except in strictly conforming C
standard modes, the X/Open function <code class="code">strfmon</code> is also checked as
are <code class="code">printf_unlocked</code> and <code class="code">fprintf_unlocked</code>.
See <a class="xref" href="C-Dialect-Options.html">Options Controlling C Dialect</a>.
</p>
<p>For Objective-C dialects, <code class="code">NSString</code> (or <code class="code">__NSString__</code>) is
recognized in the same context.  Declarations including these format attributes
are parsed for correct syntax, however the result of checking of such format
strings is not yet defined, and is not carried out by this version of the
compiler.
</p>
<p>The target may also provide additional types of format checks.
See <a class="xref" href="Target-Format-Checks.html">Format Checks Specific to Particular
Target Machines</a>.
</p>
</dd>
<dt><a class="index-entry-id" id="index-Wformat_002dnonliteral-1"></a>
<a id="index-format_005farg-function-attribute"></a><span><code class="code">format_arg (<var class="var">string-index</var>)</code><a class="copiable-link" href="#index-format_005farg-function-attribute"> &para;</a></span></dt>
<dd><p>The <code class="code">format_arg</code> attribute specifies that a function takes one or
more format strings for a <code class="code">printf</code>, <code class="code">scanf</code>, <code class="code">strftime</code> or
<code class="code">strfmon</code> style function and modifies it (for example, to translate
it into another language), so the result can be passed to a
<code class="code">printf</code>, <code class="code">scanf</code>, <code class="code">strftime</code> or <code class="code">strfmon</code> style
function (with the remaining arguments to the format function the same
as they would have been for the unmodified string).  Multiple
<code class="code">format_arg</code> attributes may be applied to the same function, each
designating a distinct parameter as a format string.  For example, the
declaration:
</p>
<div class="example smallexample">
<pre class="example-preformatted">extern char *
my_dgettext (char *my_domain, const char *my_format)
      __attribute__ ((format_arg (2)));
</pre></div>

<p>causes the compiler to check the arguments in calls to a <code class="code">printf</code>,
<code class="code">scanf</code>, <code class="code">strftime</code> or <code class="code">strfmon</code> type function, whose
format string argument is a call to the <code class="code">my_dgettext</code> function, for
consistency with the format string argument <code class="code">my_format</code>.  If the
<code class="code">format_arg</code> attribute had not been specified, all the compiler
could tell in such calls to format functions would be that the format
string argument is not constant; this would generate a warning when
<samp class="option">-Wformat-nonliteral</samp> is used, but the calls could not be checked
without the attribute.
</p>
<p>In calls to a function declared with more than one <code class="code">format_arg</code>
attribute, each with a distinct argument value, the corresponding
actual function arguments are checked against all format strings
designated by the attributes.  This capability is designed to support
the GNU <code class="code">ngettext</code> family of functions.
</p>
<p>The parameter <var class="var">string-index</var> specifies which argument is the format
string argument (starting from one).  Since non-static C++ methods have
an implicit <code class="code">this</code> argument, the arguments of such methods should
be counted from two.
</p>
<p>The <code class="code">format_arg</code> attribute allows you to identify your own
functions that modify format strings, so that GCC can check the
calls to <code class="code">printf</code>, <code class="code">scanf</code>, <code class="code">strftime</code> or <code class="code">strfmon</code>
type function whose operands are a call to one of your own function.
The compiler always treats <code class="code">gettext</code>, <code class="code">dgettext</code>, and
<code class="code">dcgettext</code> in this manner except when strict ISO C support is
requested by <samp class="option">-ansi</samp> or an appropriate <samp class="option">-std</samp> option, or
<samp class="option">-ffreestanding</samp> or <samp class="option">-fno-builtin</samp>
is used.  See <a class="xref" href="C-Dialect-Options.html">Options
Controlling C Dialect</a>.
</p>
<p>For Objective-C dialects, the <code class="code">format-arg</code> attribute may refer to an
<code class="code">NSString</code> reference for compatibility with the <code class="code">format</code> attribute
above.
</p>
<p>The target may also allow additional types in <code class="code">format-arg</code> attributes.
See <a class="xref" href="Target-Format-Checks.html">Format Checks Specific to Particular
Target Machines</a>.
</p>
</dd>
<dt><a id="index-gnu_005finline-function-attribute"></a><span><code class="code">gnu_inline</code><a class="copiable-link" href="#index-gnu_005finline-function-attribute"> &para;</a></span></dt>
<dd><p>This attribute should be used with a function that is also declared
with the <code class="code">inline</code> keyword.  It directs GCC to treat the function
as if it were defined in gnu90 mode even when compiling in C99 or
gnu99 mode.
</p>
<p>If the function is declared <code class="code">extern</code>, then this definition of the
function is used only for inlining.  In no case is the function
compiled as a standalone function, not even if you take its address
explicitly.  Such an address becomes an external reference, as if you
had only declared the function, and had not defined it.  This has
almost the effect of a macro.  The way to use this is to put a
function definition in a header file with this attribute, and put
another copy of the function, without <code class="code">extern</code>, in a library
file.  The definition in the header file causes most calls to the
function to be inlined.  If any uses of the function remain, they
refer to the single copy in the library.  Note that the two
definitions of the functions need not be precisely the same, although
if they do not have the same effect your program may behave oddly.
</p>
<p>In C, if the function is neither <code class="code">extern</code> nor <code class="code">static</code>, then
the function is compiled as a standalone function, as well as being
inlined where possible.
</p>
<p>This is how GCC traditionally handled functions declared
<code class="code">inline</code>.  Since ISO C99 specifies a different semantics for
<code class="code">inline</code>, this function attribute is provided as a transition
measure and as a useful feature in its own right.  This attribute is
available in GCC 4.1.3 and later.  It is available if either of the
preprocessor macros <code class="code">__GNUC_GNU_INLINE__</code> or
<code class="code">__GNUC_STDC_INLINE__</code> are defined.  See <a class="xref" href="Inline.html">An Inline
Function is As Fast As a Macro</a>.
</p>
<p>In C++, this attribute does not depend on <code class="code">extern</code> in any way,
but it still requires the <code class="code">inline</code> keyword to enable its special
behavior.
</p>
</dd>
<dt><a id="index-hot-function-attribute"></a><span><code class="code">hot</code><a class="copiable-link" href="#index-hot-function-attribute"> &para;</a></span></dt>
<dd><p>The <code class="code">hot</code> attribute on a function is used to inform the compiler that
the function is a hot spot of the compiled program.  The function is
optimized more aggressively and on many targets it is placed into a special
subsection of the text section so all hot functions appear close together,
improving locality.
</p>
<p>When profile feedback is available, via <samp class="option">-fprofile-use</samp>, hot functions
are automatically detected and this attribute is ignored.
</p>
</dd>
<dt><a class="index-entry-id" id="index-indirect-functions"></a>
<a class="index-entry-id" id="index-functions-that-are-dynamically-resolved"></a>
<a id="index-ifunc-function-attribute"></a><span><code class="code">ifunc (&quot;<var class="var">resolver</var>&quot;)</code><a class="copiable-link" href="#index-ifunc-function-attribute"> &para;</a></span></dt>
<dd><p>The <code class="code">ifunc</code> attribute is used to mark a function as an indirect
function using the STT_GNU_IFUNC symbol type extension to the ELF
standard.  This allows the resolution of the symbol value to be
determined dynamically at load time, and an optimized version of the
routine to be selected for the particular processor or other system
characteristics determined then.  To use this attribute, first define
the implementation functions available, and a resolver function that
returns a pointer to the selected implementation function.  The
implementation functions&rsquo; declarations must match the API of the
function being implemented.  The resolver should be declared to
be a function taking no arguments and returning a pointer to
a function of the same type as the implementation.  For example:
</p>
<div class="example smallexample">
<pre class="example-preformatted">void *my_memcpy (void *dst, const void *src, size_t len)
{
  ...
  return dst;
}

static void * (*resolve_memcpy (void))(void *, const void *, size_t)
{
  return my_memcpy; // we will just always select this routine
}
</pre></div>

<p>The exported header file declaring the function the user calls would
contain:
</p>
<div class="example smallexample">
<pre class="example-preformatted">extern void *memcpy (void *, const void *, size_t);
</pre></div>

<p>allowing the user to call <code class="code">memcpy</code> as a regular function, unaware of
the actual implementation.  Finally, the indirect function needs to be
defined in the same translation unit as the resolver function:
</p>
<div class="example smallexample">
<pre class="example-preformatted">void *memcpy (void *, const void *, size_t)
     __attribute__ ((ifunc (&quot;resolve_memcpy&quot;)));
</pre></div>

<p>In C++, the <code class="code">ifunc</code> attribute takes a string that is the mangled name
of the resolver function.  A C++ resolver for a non-static member function
of class <code class="code">C</code> should be declared to return a pointer to a non-member
function taking pointer to <code class="code">C</code> as the first argument, followed by
the same arguments as of the implementation function.  G++ checks
the signatures of the two functions and issues
a <samp class="option">-Wattribute-alias</samp> warning for mismatches.  To suppress a warning
for the necessary cast from a pointer to the implementation member function
to the type of the corresponding non-member function use
the <samp class="option">-Wno-pmf-conversions</samp> option.  For example:
</p>
<div class="example smallexample">
<pre class="example-preformatted">class S
{
private:
  int debug_impl (int);
  int optimized_impl (int);

  typedef int Func (S*, int);

  static Func* resolver ();
public:

  int interface (int);
};

int S::debug_impl (int) { /* <span class="r">...</span> */ }
int S::optimized_impl (int) { /* <span class="r">...</span> */ }

S::Func* S::resolver ()
{
  int (S::*pimpl) (int)
    = getenv (&quot;DEBUG&quot;) ? &amp;S::debug_impl : &amp;S::optimized_impl;

  // Cast triggers -Wno-pmf-conversions.
  return reinterpret_cast&lt;Func*&gt;(pimpl);
}

int S::interface (int) __attribute__ ((ifunc (&quot;_ZN1S8resolverEv&quot;)));
</pre></div>

<p>Indirect functions cannot be weak.  Binutils version 2.20.1 or higher
and GNU C Library version 2.11.1 are required to use this feature.
</p>
</dd>
<dt><code class="code">interrupt</code></dt>
<dt><code class="code">interrupt_handler</code></dt>
<dd><p>Many GCC back ends support attributes to indicate that a function is
an interrupt handler, which tells the compiler to generate function
entry and exit sequences that differ from those from regular
functions.  The exact syntax and behavior are target-specific;
refer to the following subsections for details.
</p>
</dd>
<dt><a id="index-leaf-function-attribute"></a><span><code class="code">leaf</code><a class="copiable-link" href="#index-leaf-function-attribute"> &para;</a></span></dt>
<dd><p>Calls to external functions with this attribute must return to the
current compilation unit only by return or by exception handling.  In
particular, a leaf function is not allowed to invoke callback functions
passed to it from the current compilation unit, directly call functions
exported by the unit, or <code class="code">longjmp</code> into the unit.  Leaf functions
might still call functions from other compilation units and thus they
are not necessarily leaf in the sense that they contain no function
calls at all.
</p>
<p>The attribute is intended for library functions to improve dataflow
analysis.  The compiler takes the hint that any data not escaping the
current compilation unit cannot be used or modified by the leaf
function.  For example, the <code class="code">sin</code> function is a leaf function, but
<code class="code">qsort</code> is not.
</p>
<p>Note that leaf functions might indirectly run a signal handler defined
in the current compilation unit that uses static variables.  Similarly,
when lazy symbol resolution is in effect, leaf functions might invoke
indirect functions whose resolver function or implementation function is
defined in the current compilation unit and uses static variables.  There
is no standard-compliant way to write such a signal handler, resolver
function, or implementation function, and the best that you can do is to
remove the <code class="code">leaf</code> attribute or mark all such static variables
<code class="code">volatile</code>.  Lastly, for ELF-based systems that support symbol
interposition, care should be taken that functions defined in the
current compilation unit do not unexpectedly interpose other symbols
based on the defined standards mode and defined feature test macros;
otherwise an inadvertent callback would be added.
</p>
<p>The attribute has no effect on functions defined within the current
compilation unit.  This is to allow easy merging of multiple compilation
units into one, for example, by using the link-time optimization.  For
this reason the attribute is not allowed on types to annotate indirect
calls.
</p>
</dd>
<dt><a class="index-entry-id" id="index-functions-that-behave-like-malloc"></a>
<a id="index-malloc-function-attribute"></a><span><code class="code">malloc</code><a class="copiable-link" href="#index-malloc-function-attribute"> &para;</a></span></dt>
<dt><code class="code">malloc (<var class="var">deallocator</var>)</code></dt>
<dt><code class="code">malloc (<var class="var">deallocator</var>, <var class="var">ptr-index</var>)</code></dt>
<dd><p>Attribute <code class="code">malloc</code> indicates that a function is <code class="code">malloc</code>-like,
i.e., that the pointer <var class="var">P</var> returned by the function cannot alias any
other pointer valid when the function returns, and moreover no
pointers to valid objects occur in any storage addressed by <var class="var">P</var>. In
addition, GCC predicts that a function with the attribute returns
non-null in most cases.
</p>
<p>Independently, the form of the attribute with one or two arguments
associates <code class="code">deallocator</code> as a suitable deallocation function for
pointers returned from the <code class="code">malloc</code>-like function.  <var class="var">ptr-index</var>
denotes the positional argument to which when the pointer is passed in
calls to <code class="code">deallocator</code> has the effect of deallocating it.
</p>
<p>Using the attribute with no arguments is designed to improve optimization
by relying on the aliasing property it implies.  Functions like <code class="code">malloc</code>
and <code class="code">calloc</code> have this property because they return a pointer to
uninitialized or zeroed-out, newly obtained storage.  However, functions
like <code class="code">realloc</code> do not have this property, as they may return pointers
to storage containing pointers to existing objects.  Additionally, since
all such functions are assumed to return null only infrequently, callers
can be optimized based on that assumption.
</p>
<p>Associating a function with a <var class="var">deallocator</var> helps detect calls to
mismatched allocation and deallocation functions and diagnose them under
the control of options such as <samp class="option">-Wmismatched-dealloc</samp>.  It also
makes it possible to diagnose attempts to deallocate objects that were not
allocated dynamically, by <samp class="option">-Wfree-nonheap-object</samp>.  To indicate
that an allocation function both satisifies the nonaliasing property and
has a deallocator associated with it, both the plain form of the attribute
and the one with the <var class="var">deallocator</var> argument must be used.  The same
function can be both an allocator and a deallocator.  Since inlining one
of the associated functions but not the other could result in apparent
mismatches, this form of attribute <code class="code">malloc</code> is not accepted on inline
functions.  For the same reason, using the attribute prevents both
the allocation and deallocation functions from being expanded inline.
</p>
<p>For example, besides stating that the functions return pointers that do
not alias any others, the following declarations make <code class="code">fclose</code>
a suitable deallocator for pointers returned from all functions except
<code class="code">popen</code>, and <code class="code">pclose</code> as the only suitable deallocator for
pointers returned from <code class="code">popen</code>.  The deallocator functions must
be declared before they can be referenced in the attribute.
</p>
<div class="example smallexample">
<pre class="example-preformatted">int fclose (FILE*);
int pclose (FILE*);

__attribute__ ((malloc, malloc (fclose, 1)))
  FILE* fdopen (int, const char*);
__attribute__ ((malloc, malloc (fclose, 1)))
  FILE* fopen (const char*, const char*);
__attribute__ ((malloc, malloc (fclose, 1)))
  FILE* fmemopen(void *, size_t, const char *);
__attribute__ ((malloc, malloc (pclose, 1)))
  FILE* popen (const char*, const char*);
__attribute__ ((malloc, malloc (fclose, 1)))
  FILE* tmpfile (void);
</pre></div>

<p>The warnings guarded by <samp class="option">-fanalyzer</samp> respect allocation and
deallocation pairs marked with the <code class="code">malloc</code>.  In particular:
</p>
<ul class="itemize mark-bullet">
<li>The analyzer emits a <samp class="option">-Wanalyzer-mismatching-deallocation</samp>
diagnostic if there is an execution path in which the result of an
allocation call is passed to a different deallocator.

</li><li>The analyzer emits a <samp class="option">-Wanalyzer-double-free</samp>
diagnostic if there is an execution path in which a value is passed
more than once to a deallocation call.

</li><li>The analyzer considers the possibility that an allocation function
could fail and return null.  If there are
execution paths in which an unchecked result of an allocation call is
dereferenced or passed to a function requiring a non-null argument,
it emits
<samp class="option">-Wanalyzer-possible-null-dereference</samp> and
<samp class="option">-Wanalyzer-possible-null-argument</samp> diagnostics.
If the allocator always returns non-null, use
<code class="code">__attribute__ ((returns_nonnull))</code> to suppress these warnings.
For example:
<div class="example smallexample">
<pre class="example-preformatted">char *xstrdup (const char *)
  __attribute__((malloc (free), returns_nonnull));
</pre></div>

</li><li>The analyzer emits a <samp class="option">-Wanalyzer-use-after-free</samp>
diagnostic if there is an execution path in which the memory passed
by pointer to a deallocation call is used after the deallocation.

</li><li>The analyzer emits a <samp class="option">-Wanalyzer-malloc-leak</samp> diagnostic if
there is an execution path in which the result of an allocation call
is leaked (without being passed to the deallocation function).

</li><li>The analyzer emits a <samp class="option">-Wanalyzer-free-of-non-heap</samp> diagnostic
if a deallocation function is used on a global or on-stack variable.

</li></ul>

<p>The analyzer assumes that deallocators can gracefully handle the null
pointer.  If this is not the case, the deallocator can be marked with
<code class="code">__attribute__((nonnull))</code> so that <samp class="option">-fanalyzer</samp> can emit
a <samp class="option">-Wanalyzer-possible-null-argument</samp> diagnostic for code paths
in which the deallocator is called with null.
</p>
</dd>
<dt><a id="index-no_005ficf-function-attribute"></a><span><code class="code">no_icf</code><a class="copiable-link" href="#index-no_005ficf-function-attribute"> &para;</a></span></dt>
<dd><p>This function attribute prevents a functions from being merged with another
semantically equivalent function.
</p>
</dd>
<dt><a class="index-entry-id" id="index-finstrument_002dfunctions-1"></a>
<a class="index-entry-id" id="index-p-1"></a>
<a class="index-entry-id" id="index-pg-1"></a>
<a id="index-no_005finstrument_005ffunction-function-attribute"></a><span><code class="code">no_instrument_function</code><a class="copiable-link" href="#index-no_005finstrument_005ffunction-function-attribute"> &para;</a></span></dt>
<dd><p>If any of <samp class="option">-finstrument-functions</samp>, <samp class="option">-p</samp>, or <samp class="option">-pg</samp> are 
given, profiling function calls are
generated at entry and exit of most user-compiled functions.
Functions with this attribute are not so instrumented.
</p>
</dd>
<dt><a id="index-no_005fprofile_005finstrument_005ffunction-function-attribute"></a><span><code class="code">no_profile_instrument_function</code><a class="copiable-link" href="#index-no_005fprofile_005finstrument_005ffunction-function-attribute"> &para;</a></span></dt>
<dd><p>The <code class="code">no_profile_instrument_function</code> attribute on functions is used
to inform the compiler that it should not process any profile feedback based
optimization code instrumentation.
</p>
</dd>
<dt><a id="index-no_005freorder-function-attribute"></a><span><code class="code">no_reorder</code><a class="copiable-link" href="#index-no_005freorder-function-attribute"> &para;</a></span></dt>
<dd><p>Do not reorder functions or variables marked <code class="code">no_reorder</code>
against each other or top level assembler statements the executable.
The actual order in the program will depend on the linker command
line. Static variables marked like this are also not removed.
This has a similar effect
as the <samp class="option">-fno-toplevel-reorder</samp> option, but only applies to the
marked symbols.
</p>
</dd>
<dt><a id="index-no_005fsanitize-function-attribute"></a><span><code class="code">no_sanitize (&quot;<var class="var">sanitize_option</var>&quot;)</code><a class="copiable-link" href="#index-no_005fsanitize-function-attribute"> &para;</a></span></dt>
<dd><p>The <code class="code">no_sanitize</code> attribute on functions is used
to inform the compiler that it should not do sanitization of any option
mentioned in <var class="var">sanitize_option</var>.  A list of values acceptable by
the <samp class="option">-fsanitize</samp> option can be provided.
</p>
<div class="example smallexample">
<pre class="example-preformatted">void __attribute__ ((no_sanitize (&quot;alignment&quot;, &quot;object-size&quot;)))
f () { /* <span class="r">Do something.</span> */; }
void __attribute__ ((no_sanitize (&quot;alignment,object-size&quot;)))
g () { /* <span class="r">Do something.</span> */; }
</pre></div>

</dd>
<dt><a id="index-no_005fsanitize_005faddress-function-attribute"></a><span><code class="code">no_sanitize_address</code><a class="copiable-link" href="#index-no_005fsanitize_005faddress-function-attribute"> &para;</a></span></dt>
<dt><code class="code">no_address_safety_analysis</code></dt>
<dd><p>The <code class="code">no_sanitize_address</code> attribute on functions is used
to inform the compiler that it should not instrument memory accesses
in the function when compiling with the <samp class="option">-fsanitize=address</samp> option.
The <code class="code">no_address_safety_analysis</code> is a deprecated alias of the
<code class="code">no_sanitize_address</code> attribute, new code should use
<code class="code">no_sanitize_address</code>.
</p>
</dd>
<dt><a id="index-no_005fsanitize_005fthread-function-attribute"></a><span><code class="code">no_sanitize_thread</code><a class="copiable-link" href="#index-no_005fsanitize_005fthread-function-attribute"> &para;</a></span></dt>
<dd><p>The <code class="code">no_sanitize_thread</code> attribute on functions is used
to inform the compiler that it should not instrument memory accesses
in the function when compiling with the <samp class="option">-fsanitize=thread</samp> option.
</p>
</dd>
<dt><a id="index-no_005fsanitize_005fundefined-function-attribute"></a><span><code class="code">no_sanitize_undefined</code><a class="copiable-link" href="#index-no_005fsanitize_005fundefined-function-attribute"> &para;</a></span></dt>
<dd><p>The <code class="code">no_sanitize_undefined</code> attribute on functions is used
to inform the compiler that it should not check for undefined behavior
in the function when compiling with the <samp class="option">-fsanitize=undefined</samp> option.
</p>
</dd>
<dt><a id="index-no_005fsanitize_005fcoverage-function-attribute"></a><span><code class="code">no_sanitize_coverage</code><a class="copiable-link" href="#index-no_005fsanitize_005fcoverage-function-attribute"> &para;</a></span></dt>
<dd><p>The <code class="code">no_sanitize_coverage</code> attribute on functions is used
to inform the compiler that it should not do coverage-guided
fuzzing code instrumentation (<samp class="option">-fsanitize-coverage</samp>).
</p>
</dd>
<dt><a class="index-entry-id" id="index-fsplit_002dstack-1"></a>
<a id="index-no_005fsplit_005fstack-function-attribute"></a><span><code class="code">no_split_stack</code><a class="copiable-link" href="#index-no_005fsplit_005fstack-function-attribute"> &para;</a></span></dt>
<dd><p>If <samp class="option">-fsplit-stack</samp> is given, functions have a small
prologue which decides whether to split the stack.  Functions with the
<code class="code">no_split_stack</code> attribute do not have that prologue, and thus
may run with only a small amount of stack space available.
</p>
</dd>
<dt><a id="index-no_005fstack_005flimit-function-attribute"></a><span><code class="code">no_stack_limit</code><a class="copiable-link" href="#index-no_005fstack_005flimit-function-attribute"> &para;</a></span></dt>
<dd><p>This attribute locally overrides the <samp class="option">-fstack-limit-register</samp>
and <samp class="option">-fstack-limit-symbol</samp> command-line options; it has the effect
of disabling stack limit checking in the function it applies to.
</p>
</dd>
<dt><a id="index-noclone-function-attribute"></a><span><code class="code">noclone</code><a class="copiable-link" href="#index-noclone-function-attribute"> &para;</a></span></dt>
<dd><p>This function attribute prevents a function from being considered for
cloning&mdash;a mechanism that produces specialized copies of functions
and which is (currently) performed by interprocedural constant
propagation.
</p>
</dd>
<dt><a id="index-noinline-function-attribute"></a><span><code class="code">noinline</code><a class="copiable-link" href="#index-noinline-function-attribute"> &para;</a></span></dt>
<dd><p>This function attribute prevents a function from being considered for
inlining.
If the function does not have side effects, there are optimizations
other than inlining that cause function calls to be optimized away,
although the function call is live.  To keep such calls from being
optimized away, put
</p><div class="example smallexample">
<pre class="example-preformatted">asm (&quot;&quot;);
</pre></div>

<p>(see <a class="pxref" href="Extended-Asm.html">Extended Asm - Assembler Instructions with C Expression Operands</a>) in the called function, to serve as a special
side effect.
</p>
</dd>
<dt><a id="index-noipa-function-attribute"></a><span><code class="code">noipa</code><a class="copiable-link" href="#index-noipa-function-attribute"> &para;</a></span></dt>
<dd><p>Disable interprocedural optimizations between the function with this
attribute and its callers, as if the body of the function is not available
when optimizing callers and the callers are unavailable when optimizing
the body.  This attribute implies <code class="code">noinline</code>, <code class="code">noclone</code> and
<code class="code">no_icf</code> attributes.    However, this attribute is not equivalent
to a combination of other attributes, because its purpose is to suppress
existing and future optimizations employing interprocedural analysis,
including those that do not have an attribute suitable for disabling
them individually.  This attribute is supported mainly for the purpose
of testing the compiler.
</p>
</dd>
<dt><a class="index-entry-id" id="index-functions-with-non_002dnull-pointer-arguments"></a>
<a id="index-nonnull-function-attribute"></a><span><code class="code">nonnull</code><a class="copiable-link" href="#index-nonnull-function-attribute"> &para;</a></span></dt>
<dt><code class="code">nonnull (<var class="var">arg-index</var>, &hellip;)</code></dt>
<dd><p>The <code class="code">nonnull</code> attribute may be applied to a function that takes at
least one argument of a pointer type.  It indicates that the referenced
arguments must be non-null pointers.  For instance, the declaration:
</p>
<div class="example smallexample">
<pre class="example-preformatted">extern void *
my_memcpy (void *dest, const void *src, size_t len)
        __attribute__((nonnull (1, 2)));
</pre></div>

<p>informs the compiler that, in calls to <code class="code">my_memcpy</code>, arguments
<var class="var">dest</var> and <var class="var">src</var> must be non-null.
</p>
<p>The attribute has an effect both on functions calls and function definitions.
</p>
<p>For function calls:
</p><ul class="itemize mark-bullet">
<li>If the compiler determines that a null pointer is
passed in an argument slot marked as non-null, and the
<samp class="option">-Wnonnull</samp> option is enabled, a warning is issued.
See <a class="xref" href="Warning-Options.html">Options to Request or Suppress Warnings</a>.
</li><li>The <samp class="option">-fisolate-erroneous-paths-attribute</samp> option can be
specified to have GCC transform calls with null arguments to non-null
functions into traps.  See <a class="xref" href="Optimize-Options.html">Options That Control Optimization</a>.
</li><li>The compiler may also perform optimizations based on the
knowledge that certain function arguments cannot be null.  These
optimizations can be disabled by the
<samp class="option">-fno-delete-null-pointer-checks</samp> option. See <a class="xref" href="Optimize-Options.html">Options That Control Optimization</a>.
</li></ul>

<p>For function definitions:
</p><ul class="itemize mark-bullet">
<li>If the compiler determines that a function parameter that is
marked with nonnull is compared with null, and
<samp class="option">-Wnonnull-compare</samp> option is enabled, a warning is issued.
See <a class="xref" href="Warning-Options.html">Options to Request or Suppress Warnings</a>.
</li><li>The compiler may also perform optimizations based on the
knowledge that <code class="code">nonnull</code> parameters cannot be null.  This can
currently not be disabled other than by removing the nonnull
attribute.
</li></ul>

<p>If no <var class="var">arg-index</var> is given to the <code class="code">nonnull</code> attribute,
all pointer arguments are marked as non-null.  To illustrate, the
following declaration is equivalent to the previous example:
</p>
<div class="example smallexample">
<pre class="example-preformatted">extern void *
my_memcpy (void *dest, const void *src, size_t len)
        __attribute__((nonnull));
</pre></div>

</dd>
<dt><a id="index-noplt-function-attribute"></a><span><code class="code">noplt</code><a class="copiable-link" href="#index-noplt-function-attribute"> &para;</a></span></dt>
<dd><p>The <code class="code">noplt</code> attribute is the counterpart to option <samp class="option">-fno-plt</samp>.
Calls to functions marked with this attribute in position-independent code
do not use the PLT.
</p>
<div class="example smallexample">
<div class="group"><pre class="example-preformatted">/* Externally defined function foo.  */
int foo () __attribute__ ((noplt));

int
main (/* <span class="r">...</span> */)
{
  /* <span class="r">...</span> */
  foo ();
  /* <span class="r">...</span> */
}
</pre></div></div>

<p>The <code class="code">noplt</code> attribute on function <code class="code">foo</code>
tells the compiler to assume that
the function <code class="code">foo</code> is externally defined and that the call to
<code class="code">foo</code> must avoid the PLT
in position-independent code.
</p>
<p>In position-dependent code, a few targets also convert calls to
functions that are marked to not use the PLT to use the GOT instead.
</p>
</dd>
<dt><a class="index-entry-id" id="index-functions-that-never-return"></a>
<a id="index-noreturn-function-attribute"></a><span><code class="code">noreturn</code><a class="copiable-link" href="#index-noreturn-function-attribute"> &para;</a></span></dt>
<dd><p>A few standard library functions, such as <code class="code">abort</code> and <code class="code">exit</code>,
cannot return.  GCC knows this automatically.  Some programs define
their own functions that never return.  You can declare them
<code class="code">noreturn</code> to tell the compiler this fact.  For example,
</p>
<div class="example smallexample">
<div class="group"><pre class="example-preformatted">void fatal () __attribute__ ((noreturn));

void
fatal (/* <span class="r">...</span> */)
{
  /* <span class="r">...</span> */ /* <span class="r">Print error message.</span> */ /* <span class="r">...</span> */
  exit (1);
}
</pre></div></div>

<p>The <code class="code">noreturn</code> keyword tells the compiler to assume that
<code class="code">fatal</code> cannot return.  It can then optimize without regard to what
would happen if <code class="code">fatal</code> ever did return.  This makes slightly
better code.  More importantly, it helps avoid spurious warnings of
uninitialized variables.
</p>
<p>The <code class="code">noreturn</code> keyword does not affect the exceptional path when that
applies: a <code class="code">noreturn</code>-marked function may still return to the caller
by throwing an exception or calling <code class="code">longjmp</code>.
</p>
<p>In order to preserve backtraces, GCC will never turn calls to
<code class="code">noreturn</code> functions into tail calls.
</p>
<p>Do not assume that registers saved by the calling function are
restored before calling the <code class="code">noreturn</code> function.
</p>
<p>It does not make sense for a <code class="code">noreturn</code> function to have a return
type other than <code class="code">void</code>.
</p>
</dd>
<dt><a id="index-nothrow-function-attribute"></a><span><code class="code">nothrow</code><a class="copiable-link" href="#index-nothrow-function-attribute"> &para;</a></span></dt>
<dd><p>The <code class="code">nothrow</code> attribute is used to inform the compiler that a
function cannot throw an exception.  For example, most functions in
the standard C library can be guaranteed not to throw an exception
with the notable exceptions of <code class="code">qsort</code> and <code class="code">bsearch</code> that
take function pointer arguments.
</p>
</dd>
<dt><a id="index-optimize-function-attribute"></a><span><code class="code">optimize (<var class="var">level</var>, &hellip;)</code><a class="copiable-link" href="#index-optimize-function-attribute"> &para;</a></span></dt>
<dt><code class="code">optimize (<var class="var">string</var>, &hellip;)</code></dt>
<dd><p>The <code class="code">optimize</code> attribute is used to specify that a function is to
be compiled with different optimization options than specified on the
command line.  The optimize attribute arguments of a function behave
as if appended to the command-line.
</p>
<p>Valid arguments are constant non-negative integers and
strings.  Each numeric argument specifies an optimization <var class="var">level</var>.
Each <var class="var">string</var> argument consists of one or more comma-separated
substrings.  Each substring that begins with the letter <code class="code">O</code> refers
to an optimization option such as <samp class="option">-O0</samp> or <samp class="option">-Os</samp>.  Other
substrings are taken as suffixes to the <code class="code">-f</code> prefix jointly
forming the name of an optimization option.  See <a class="xref" href="Optimize-Options.html">Options That Control Optimization</a>.
</p>
<p>&lsquo;<samp class="samp">#pragma GCC optimize</samp>&rsquo; can be used to set optimization options
for more than one function.  See <a class="xref" href="Function-Specific-Option-Pragmas.html">Function Specific Option Pragmas</a>,
for details about the pragma.
</p>
<p>Providing multiple strings as arguments separated by commas to specify
multiple options is equivalent to separating the option suffixes with
a comma (&lsquo;<samp class="samp">,</samp>&rsquo;) within a single string.  Spaces are not permitted
within the strings.
</p>
<p>Not every optimization option that starts with the <var class="var">-f</var> prefix
specified by the attribute necessarily has an effect on the function.
The <code class="code">optimize</code> attribute should be used for debugging purposes only.
It is not suitable in production code.
</p>
</dd>
<dt><a class="index-entry-id" id="index-extra-NOP-instructions-at-the-function-entry-point"></a>
<a id="index-patchable_005ffunction_005fentry-function-attribute"></a><span><code class="code">patchable_function_entry</code><a class="copiable-link" href="#index-patchable_005ffunction_005fentry-function-attribute"> &para;</a></span></dt>
<dd><p>In case the target&rsquo;s text segment can be made writable at run time by
any means, padding the function entry with a number of NOPs can be
used to provide a universal tool for instrumentation.
</p>
<p>The <code class="code">patchable_function_entry</code> function attribute can be used to
change the number of NOPs to any desired value.  The two-value syntax
is the same as for the command-line switch
<samp class="option">-fpatchable-function-entry=N,M</samp>, generating <var class="var">N</var> NOPs, with
the function entry point before the <var class="var">M</var>th NOP instruction.
<var class="var">M</var> defaults to 0 if omitted e.g. function entry point is before
the first NOP.
</p>
<p>If patchable function entries are enabled globally using the command-line
option <samp class="option">-fpatchable-function-entry=N,M</samp>, then you must disable
instrumentation on all functions that are part of the instrumentation
framework with the attribute <code class="code">patchable_function_entry (0)</code>
to prevent recursion.
</p>
</dd>
<dt><a class="index-entry-id" id="index-functions-that-have-no-side-effects-1"></a>
<a id="index-pure-function-attribute"></a><span><code class="code">pure</code><a class="copiable-link" href="#index-pure-function-attribute"> &para;</a></span></dt>
<dd>
<p>Calls to functions that have no observable effects on the state of
the program other than to return a value may lend themselves to optimizations
such as common subexpression elimination.  Declaring such functions with
the <code class="code">pure</code> attribute allows GCC to avoid emitting some calls in repeated
invocations of the function with the same argument values.
</p>
<p>The <code class="code">pure</code> attribute prohibits a function from modifying the state
of the program that is observable by means other than inspecting
the function&rsquo;s return value.  However, functions declared with the <code class="code">pure</code>
attribute can safely read any non-volatile objects, and modify the value of
objects in a way that does not affect their return value or the observable
state of the program.
</p>
<p>For example,
</p>
<div class="example smallexample">
<pre class="example-preformatted">int hash (char *) __attribute__ ((pure));
</pre></div>

<p>tells GCC that subsequent calls to the function <code class="code">hash</code> with the same
string can be replaced by the result of the first call provided the state
of the program observable by <code class="code">hash</code>, including the contents of the array
itself, does not change in between.  Even though <code class="code">hash</code> takes a non-const
pointer argument it must not modify the array it points to, or any other object
whose value the rest of the program may depend on.  However, the caller may
safely change the contents of the array between successive calls to
the function (doing so disables the optimization).  The restriction also
applies to member objects referenced by the <code class="code">this</code> pointer in C++
non-static member functions.
</p>
<p>Some common examples of pure functions are <code class="code">strlen</code> or <code class="code">memcmp</code>.
Interesting non-pure functions are functions with infinite loops or those
depending on volatile memory or other system resource, that may change between
consecutive calls (such as the standard C <code class="code">feof</code> function in
a multithreading environment).
</p>
<p>The <code class="code">pure</code> attribute imposes similar but looser restrictions on
a function&rsquo;s definition than the <code class="code">const</code> attribute: <code class="code">pure</code>
allows the function to read any non-volatile memory, even if it changes
in between successive invocations of the function.  Declaring the same
function with both the <code class="code">pure</code> and the <code class="code">const</code> attribute is
diagnosed.  Because a pure function cannot have any observable side
effects it does not make sense for such a function to return <code class="code">void</code>.
Declaring such a function is diagnosed.
</p>
</dd>
<dt><a id="index-returns_005fnonnull-function-attribute"></a><span><code class="code">returns_nonnull</code><a class="copiable-link" href="#index-returns_005fnonnull-function-attribute"> &para;</a></span></dt>
<dd><p>The <code class="code">returns_nonnull</code> attribute specifies that the function
return value should be a non-null pointer.  For instance, the declaration:
</p>
<div class="example smallexample">
<pre class="example-preformatted">extern void *
mymalloc (size_t len) __attribute__((returns_nonnull));
</pre></div>

<p>lets the compiler optimize callers based on the knowledge
that the return value will never be null.
</p>
</dd>
<dt><a class="index-entry-id" id="index-functions-that-return-more-than-once"></a>
<a id="index-returns_005ftwice-function-attribute"></a><span><code class="code">returns_twice</code><a class="copiable-link" href="#index-returns_005ftwice-function-attribute"> &para;</a></span></dt>
<dd><p>The <code class="code">returns_twice</code> attribute tells the compiler that a function may
return more than one time.  The compiler ensures that all registers
are dead before calling such a function and emits a warning about
the variables that may be clobbered after the second return from the
function.  Examples of such functions are <code class="code">setjmp</code> and <code class="code">vfork</code>.
The <code class="code">longjmp</code>-like counterpart of such function, if any, might need
to be marked with the <code class="code">noreturn</code> attribute.
</p>
</dd>
<dt><a class="index-entry-id" id="index-functions-in-arbitrary-sections"></a>
<a id="index-section-function-attribute"></a><span><code class="code">section (&quot;<var class="var">section-name</var>&quot;)</code><a class="copiable-link" href="#index-section-function-attribute"> &para;</a></span></dt>
<dd><p>Normally, the compiler places the code it generates in the <code class="code">text</code> section.
Sometimes, however, you need additional sections, or you need certain
particular functions to appear in special sections.  The <code class="code">section</code>
attribute specifies that a function lives in a particular section.
For example, the declaration:
</p>
<div class="example smallexample">
<pre class="example-preformatted">extern void foobar (void) __attribute__ ((section (&quot;bar&quot;)));
</pre></div>

<p>puts the function <code class="code">foobar</code> in the <code class="code">bar</code> section.
</p>
<p>Some file formats do not support arbitrary sections so the <code class="code">section</code>
attribute is not available on all platforms.
If you need to map the entire contents of a module to a particular
section, consider using the facilities of the linker instead.
</p>
</dd>
<dt><a id="index-sentinel-function-attribute"></a><span><code class="code">sentinel</code><a class="copiable-link" href="#index-sentinel-function-attribute"> &para;</a></span></dt>
<dt><code class="code">sentinel (<var class="var">position</var>)</code></dt>
<dd><p>This function attribute indicates that an argument in a call to the function
is expected to be an explicit <code class="code">NULL</code>.  The attribute is only valid on
variadic functions.  By default, the sentinel is expected to be the last
argument of the function call.  If the optional <var class="var">position</var> argument
is specified to the attribute, the sentinel must be located at
<var class="var">position</var> counting backwards from the end of the argument list.
</p>
<div class="example smallexample">
<pre class="example-preformatted">__attribute__ ((sentinel))
is equivalent to
__attribute__ ((sentinel(0)))
</pre></div>

<p>The attribute is automatically set with a position of 0 for the built-in
functions <code class="code">execl</code> and <code class="code">execlp</code>.  The built-in function
<code class="code">execle</code> has the attribute set with a position of 1.
</p>
<p>A valid <code class="code">NULL</code> in this context is defined as zero with any object
pointer type.  If your system defines the <code class="code">NULL</code> macro with
an integer type then you need to add an explicit cast.  During
installation GCC replaces the system <code class="code">&lt;stddef.h&gt;</code> header with
a copy that redefines NULL appropriately.
</p>
<p>The warnings for missing or incorrect sentinels are enabled with
<samp class="option">-Wformat</samp>.
</p>
</dd>
<dt><a id="index-simd-function-attribute"></a><span><code class="code">simd</code><a class="copiable-link" href="#index-simd-function-attribute"> &para;</a></span></dt>
<dt><code class="code">simd(&quot;<var class="var">mask</var>&quot;)</code></dt>
<dd><p>This attribute enables creation of one or more function versions that
can process multiple arguments using SIMD instructions from a
single invocation.  Specifying this attribute allows compiler to
assume that such versions are available at link time (provided
in the same or another translation unit).  Generated versions are
target-dependent and described in the corresponding Vector ABI document.  For
x86_64 target this document can be found
<a class="uref" href="https://sourceware.org/glibc/wiki/libmvec?action=AttachFile&amp;do=view&amp;target=VectorABI.txt">here</a><!-- /@w -->.
</p>
<p>The optional argument <var class="var">mask</var> may have the value
<code class="code">notinbranch</code> or <code class="code">inbranch</code>,
and instructs the compiler to generate non-masked or masked
clones correspondingly. By default, all clones are generated.
</p>
<p>If the attribute is specified and <code class="code">#pragma omp declare simd</code> is
present on a declaration and the <samp class="option">-fopenmp</samp> or <samp class="option">-fopenmp-simd</samp>
switch is specified, then the attribute is ignored.
</p>
</dd>
<dt><a id="index-stack_005fprotect-function-attribute"></a><span><code class="code">stack_protect</code><a class="copiable-link" href="#index-stack_005fprotect-function-attribute"> &para;</a></span></dt>
<dd><p>This attribute adds stack protection code to the function if 
flags <samp class="option">-fstack-protector</samp>, <samp class="option">-fstack-protector-strong</samp>
or <samp class="option">-fstack-protector-explicit</samp> are set.
</p>
</dd>
<dt><a id="index-no_005fstack_005fprotector-function-attribute"></a><span><code class="code">no_stack_protector</code><a class="copiable-link" href="#index-no_005fstack_005fprotector-function-attribute"> &para;</a></span></dt>
<dd><p>This attribute prevents stack protection code for the function.
</p>
</dd>
<dt><a id="index-target-function-attribute"></a><span><code class="code">target (<var class="var">string</var>, &hellip;)</code><a class="copiable-link" href="#index-target-function-attribute"> &para;</a></span></dt>
<dd><p>Multiple target back ends implement the <code class="code">target</code> attribute
to specify that a function is to
be compiled with different target options than specified on the
command line.  The original target command-line options are ignored.
One or more strings can be provided as arguments.
Each string consists of one or more comma-separated suffixes to
the <code class="code">-m</code> prefix jointly forming the name of a machine-dependent
option.  See <a class="xref" href="Submodel-Options.html">Machine-Dependent Options</a>.
</p>
<p>The <code class="code">target</code> attribute can be used for instance to have a function
compiled with a different ISA (instruction set architecture) than the
default.  &lsquo;<samp class="samp">#pragma GCC target</samp>&rsquo; can be used to specify target-specific
options for more than one function.  See <a class="xref" href="Function-Specific-Option-Pragmas.html">Function Specific Option Pragmas</a>,
for details about the pragma.
</p>
<p>For instance, on an x86, you could declare one function with the
<code class="code">target(&quot;sse4.1,arch=core2&quot;)</code> attribute and another with
<code class="code">target(&quot;sse4a,arch=amdfam10&quot;)</code>.  This is equivalent to
compiling the first function with <samp class="option">-msse4.1</samp> and
<samp class="option">-march=core2</samp> options, and the second function with
<samp class="option">-msse4a</samp> and <samp class="option">-march=amdfam10</samp> options.  It is up to you
to make sure that a function is only invoked on a machine that
supports the particular ISA it is compiled for (for example by using
<code class="code">cpuid</code> on x86 to determine what feature bits and architecture
family are used).
</p>
<div class="example smallexample">
<pre class="example-preformatted">int core2_func (void) __attribute__ ((__target__ (&quot;arch=core2&quot;)));
int sse3_func (void) __attribute__ ((__target__ (&quot;sse3&quot;)));
</pre></div>

<p>Providing multiple strings as arguments separated by commas to specify
multiple options is equivalent to separating the option suffixes with
a comma (&lsquo;<samp class="samp">,</samp>&rsquo;) within a single string.  Spaces are not permitted
within the strings.
</p>
<p>The options supported are specific to each target; refer to <a class="ref" href="x86-Function-Attributes.html">x86 Function Attributes</a>, <a class="ref" href="PowerPC-Function-Attributes.html">PowerPC Function Attributes</a>,
<a class="ref" href="ARM-Function-Attributes.html">ARM Function Attributes</a>, <a class="ref" href="AArch64-Function-Attributes.html">AArch64 Function Attributes</a>,
<a class="ref" href="Nios-II-Function-Attributes.html">Nios II Function Attributes</a>, and <a class="ref" href="S_002f390-Function-Attributes.html">S/390 Function Attributes</a>
for details.
</p>
</dd>
<dt><a id="index-symver-function-attribute"></a><span><code class="code">symver (&quot;<var class="var">name2</var>@<var class="var">nodename</var>&quot;)</code><a class="copiable-link" href="#index-symver-function-attribute"> &para;</a></span></dt>
<dd><p>On ELF targets this attribute creates a symbol version.  The <var class="var">name2</var> part
of the parameter is the actual name of the symbol by which it will be
externally referenced.  The <code class="code">nodename</code> portion should be the name of a
node specified in the version script supplied to the linker when building a
shared library.  Versioned symbol must be defined and must be exported with
default visibility.
</p>
<div class="example smallexample">
<pre class="example-preformatted">__attribute__ ((__symver__ (&quot;foo@VERS_1&quot;))) int
foo_v1 (void)
{
}
</pre></div>

<p>Will produce a <code class="code">.symver foo_v1, foo@VERS_1</code> directive in the assembler
output. 
</p>
<p>One can also define multiple version for a given symbol
(starting from binutils 2.35).
</p>
<div class="example smallexample">
<pre class="example-preformatted">__attribute__ ((__symver__ (&quot;foo@VERS_2&quot;), __symver__ (&quot;foo@VERS_3&quot;)))
int symver_foo_v1 (void)
{
}
</pre></div>

<p>This example creates a symbol name <code class="code">symver_foo_v1</code>
which will be version <code class="code">VERS_2</code> and <code class="code">VERS_3</code> of <code class="code">foo</code>.
</p>
<p>If you have an older release of binutils, then symbol alias needs to
be used:
</p>
<div class="example smallexample">
<pre class="example-preformatted">__attribute__ ((__symver__ (&quot;foo@VERS_2&quot;)))
int foo_v1 (void)
{
  return 0;
}

__attribute__ ((__symver__ (&quot;foo@VERS_3&quot;)))
__attribute__ ((alias (&quot;foo_v1&quot;)))
int symver_foo_v1 (void);
</pre></div>

<p>Finally if the parameter is <code class="code">&quot;<var class="var">name2</var>@@<var class="var">nodename</var>&quot;</code> then in
addition to creating a symbol version (as if
<code class="code">&quot;<var class="var">name2</var>@<var class="var">nodename</var>&quot;</code> was used) the version will be also used
to resolve <var class="var">name2</var> by the linker.
</p>
</dd>
<dt><a id="index-tainted_005fargs-function-attribute"></a><span><code class="code">tainted_args</code><a class="copiable-link" href="#index-tainted_005fargs-function-attribute"> &para;</a></span></dt>
<dd><p>The <code class="code">tainted_args</code> attribute is used to specify that a function is called
in a way that requires sanitization of its arguments, such as a system
call in an operating system kernel.  Such a function can be considered part
of the &ldquo;attack surface&rdquo; of the program.  The attribute can be used both
on function declarations, and on field declarations containing function
pointers.  In the latter case, any function used as an initializer of
such a callback field will be treated as being called with tainted
arguments.
</p>
<p>The analyzer will pay particular attention to such functions when both
<samp class="option">-fanalyzer</samp> and <samp class="option">-fanalyzer-checker=taint</samp> are supplied,
potentially issuing warnings guarded by
<samp class="option">-Wanalyzer-tainted-allocation-size</samp>,
<samp class="option">-Wanalyzer-tainted-array-index</samp>,
<samp class="option">-Wanalyzer-tainted-divisor</samp>,
<samp class="option">-Wanalyzer-tainted-offset</samp>,
and <samp class="option">-Wanalyzer-tainted-size</samp>.
</p>
</dd>
<dt><a id="index-target_005fclones-function-attribute"></a><span><code class="code">target_clones (<var class="var">options</var>)</code><a class="copiable-link" href="#index-target_005fclones-function-attribute"> &para;</a></span></dt>
<dd><p>The <code class="code">target_clones</code> attribute is used to specify that a function
be cloned into multiple versions compiled with different target options
than specified on the command line.  The supported options and restrictions
are the same as for <code class="code">target</code> attribute.
</p>
<p>For instance, on an x86, you could compile a function with
<code class="code">target_clones(&quot;sse4.1,avx&quot;)</code>.  GCC creates two function clones,
one compiled with <samp class="option">-msse4.1</samp> and another with <samp class="option">-mavx</samp>.
</p>
<p>On a PowerPC, you can compile a function with
<code class="code">target_clones(&quot;cpu=power9,default&quot;)</code>.  GCC will create two
function clones, one compiled with <samp class="option">-mcpu=power9</samp> and another
with the default options.  GCC must be configured to use GLIBC 2.23 or
newer in order to use the <code class="code">target_clones</code> attribute.
</p>
<p>It also creates a resolver function (see
the <code class="code">ifunc</code> attribute above) that dynamically selects a clone
suitable for current architecture.  The resolver is created only if there
is a usage of a function with <code class="code">target_clones</code> attribute.
</p>
<p>Note that any subsequent call of a function without <code class="code">target_clone</code>
from a <code class="code">target_clone</code> caller will not lead to copying
(target clone) of the called function.
If you want to enforce such behaviour,
we recommend declaring the calling function with the <code class="code">flatten</code> attribute?
</p>
</dd>
<dt><a id="index-unused-function-attribute"></a><span><code class="code">unused</code><a class="copiable-link" href="#index-unused-function-attribute"> &para;</a></span></dt>
<dd><p>This attribute, attached to a function, means that the function is meant
to be possibly unused.  GCC does not produce a warning for this
function.
</p>
</dd>
<dt><a id="index-used-function-attribute"></a><span><code class="code">used</code><a class="copiable-link" href="#index-used-function-attribute"> &para;</a></span></dt>
<dd><p>This attribute, attached to a function, means that code must be emitted
for the function even if it appears that the function is not referenced.
This is useful, for example, when the function is referenced only in
inline assembly.
</p>
<p>When applied to a member function of a C++ class template, the
attribute also means that the function is instantiated if the
class itself is instantiated.
</p>
</dd>
<dt><a id="index-retain-function-attribute"></a><span><code class="code">retain</code><a class="copiable-link" href="#index-retain-function-attribute"> &para;</a></span></dt>
<dd><p>For ELF targets that support the GNU or FreeBSD OSABIs, this attribute
will save the function from linker garbage collection.  To support
this behavior, functions that have not been placed in specific sections
(e.g. by the <code class="code">section</code> attribute, or the <code class="code">-ffunction-sections</code>
option), will be placed in new, unique sections.
</p>
<p>This additional functionality requires Binutils version 2.36 or later.
</p>
</dd>
<dt><a id="index-visibility-function-attribute"></a><span><code class="code">visibility (&quot;<var class="var">visibility_type</var>&quot;)</code><a class="copiable-link" href="#index-visibility-function-attribute"> &para;</a></span></dt>
<dd><p>This attribute affects the linkage of the declaration to which it is attached.
It can be applied to variables (see <a class="pxref" href="Common-Variable-Attributes.html">Common Variable Attributes</a>) and types
(see <a class="pxref" href="Common-Type-Attributes.html">Common Type Attributes</a>) as well as functions.
</p>
<p>There are four supported <var class="var">visibility_type</var> values: default,
hidden, protected or internal visibility.
</p>
<div class="example smallexample">
<pre class="example-preformatted">void __attribute__ ((visibility (&quot;protected&quot;)))
f () { /* <span class="r">Do something.</span> */; }
int i __attribute__ ((visibility (&quot;hidden&quot;)));
</pre></div>

<p>The possible values of <var class="var">visibility_type</var> correspond to the
visibility settings in the ELF gABI.
</p>
<dl class="table">
<dt><code class="code">default</code></dt>
<dd><p>Default visibility is the normal case for the object file format.
This value is available for the visibility attribute to override other
options that may change the assumed visibility of entities.
</p>
<p>On ELF, default visibility means that the declaration is visible to other
modules and, in shared libraries, means that the declared entity may be
overridden.
</p>
<p>On Darwin, default visibility means that the declaration is visible to
other modules.
</p>
<p>Default visibility corresponds to &ldquo;external linkage&rdquo; in the language.
</p>
</dd>
<dt><code class="code">hidden</code></dt>
<dd><p>Hidden visibility indicates that the entity declared has a new
form of linkage, which we call &ldquo;hidden linkage&rdquo;.  Two
declarations of an object with hidden linkage refer to the same object
if they are in the same shared object.
</p>
</dd>
<dt><code class="code">internal</code></dt>
<dd><p>Internal visibility is like hidden visibility, but with additional
processor specific semantics.  Unless otherwise specified by the
psABI, GCC defines internal visibility to mean that a function is
<em class="emph">never</em> called from another module.  Compare this with hidden
functions which, while they cannot be referenced directly by other
modules, can be referenced indirectly via function pointers.  By
indicating that a function cannot be called from outside the module,
GCC may for instance omit the load of a PIC register since it is known
that the calling function loaded the correct value.
</p>
</dd>
<dt><code class="code">protected</code></dt>
<dd><p>Protected visibility is like default visibility except that it
indicates that references within the defining module bind to the
definition in that module.  That is, the declared entity cannot be
overridden by another module.
</p>
</dd>
</dl>

<p>All visibilities are supported on many, but not all, ELF targets
(supported when the assembler supports the &lsquo;<samp class="samp">.visibility</samp>&rsquo;
pseudo-op).  Default visibility is supported everywhere.  Hidden
visibility is supported on Darwin targets.
</p>
<p>The visibility attribute should be applied only to declarations that
would otherwise have external linkage.  The attribute should be applied
consistently, so that the same entity should not be declared with
different settings of the attribute.
</p>
<p>In C++, the visibility attribute applies to types as well as functions
and objects, because in C++ types have linkage.  A class must not have
greater visibility than its non-static data member types and bases,
and class members default to the visibility of their class.  Also, a
declaration without explicit visibility is limited to the visibility
of its type.
</p>
<p>In C++, you can mark member functions and static member variables of a
class with the visibility attribute.  This is useful if you know a
particular method or static member variable should only be used from
one shared object; then you can mark it hidden while the rest of the
class has default visibility.  Care must be taken to avoid breaking
the One Definition Rule; for example, it is usually not useful to mark
an inline method as hidden without marking the whole class as hidden.
</p>
<p>A C++ namespace declaration can also have the visibility attribute.
</p>
<div class="example smallexample">
<pre class="example-preformatted">namespace nspace1 __attribute__ ((visibility (&quot;protected&quot;)))
{ /* <span class="r">Do something.</span> */; }
</pre></div>

<p>This attribute applies only to the particular namespace body, not to
other definitions of the same namespace; it is equivalent to using
&lsquo;<samp class="samp">#pragma GCC visibility</samp>&rsquo; before and after the namespace
definition (see <a class="pxref" href="Visibility-Pragmas.html">Visibility Pragmas</a>).
</p>
<p>In C++, if a template argument has limited visibility, this
restriction is implicitly propagated to the template instantiation.
Otherwise, template instantiations and specializations default to the
visibility of their template.
</p>
<p>If both the template and enclosing class have explicit visibility, the
visibility from the template is used.
</p>
</dd>
<dt><a id="index-warn_005funused_005fresult-function-attribute"></a><span><code class="code">warn_unused_result</code><a class="copiable-link" href="#index-warn_005funused_005fresult-function-attribute"> &para;</a></span></dt>
<dd><p>The <code class="code">warn_unused_result</code> attribute causes a warning to be emitted
if a caller of the function with this attribute does not use its
return value.  This is useful for functions where not checking
the result is either a security problem or always a bug, such as
<code class="code">realloc</code>.
</p>
<div class="example smallexample">
<pre class="example-preformatted">int fn () __attribute__ ((warn_unused_result));
int foo ()
{
  if (fn () &lt; 0) return -1;
  fn ();
  return 0;
}
</pre></div>

<p>results in warning on line 5.
</p>
</dd>
<dt><a id="index-weak-function-attribute"></a><span><code class="code">weak</code><a class="copiable-link" href="#index-weak-function-attribute"> &para;</a></span></dt>
<dd><p>The <code class="code">weak</code> attribute causes a declaration of an external symbol
to be emitted as a weak symbol rather than a global.  This is primarily
useful in defining library functions that can be overridden in user code,
though it can also be used with non-function declarations.  The overriding
symbol must have the same type as the weak symbol.  In addition, if it
designates a variable it must also have the same size and alignment as
the weak symbol.  Weak symbols are supported for ELF targets, and also
for a.out targets when using the GNU assembler and linker.
</p>
</dd>
<dt><a id="index-weakref-function-attribute"></a><span><code class="code">weakref</code><a class="copiable-link" href="#index-weakref-function-attribute"> &para;</a></span></dt>
<dt><code class="code">weakref (&quot;<var class="var">target</var>&quot;)</code></dt>
<dd><p>The <code class="code">weakref</code> attribute marks a declaration as a weak reference.
Without arguments, it should be accompanied by an <code class="code">alias</code> attribute
naming the target symbol.  Alternatively, <var class="var">target</var> may be given as
an argument to <code class="code">weakref</code> itself, naming the target definition of
the alias.  The <var class="var">target</var> must have the same type as the declaration.
In addition, if it designates a variable it must also have the same size
and alignment as the declaration.  In either form of the declaration
<code class="code">weakref</code> implicitly marks the declared symbol as <code class="code">weak</code>.  Without
a <var class="var">target</var> given as an argument to <code class="code">weakref</code> or to <code class="code">alias</code>,
<code class="code">weakref</code> is equivalent to <code class="code">weak</code> (in that case the declaration
may be <code class="code">extern</code>).
</p>
<div class="example smallexample">
<pre class="example-preformatted">/* Given the declaration: */
extern int y (void);

/* the following... */
static int x (void) __attribute__ ((weakref (&quot;y&quot;)));

/* is equivalent to... */
static int x (void) __attribute__ ((weakref, alias (&quot;y&quot;)));

/* or, alternatively, to... */
static int x (void) __attribute__ ((weakref));
static int x (void) __attribute__ ((alias (&quot;y&quot;)));
</pre></div>

<p>A weak reference is an alias that does not by itself require a
definition to be given for the target symbol.  If the target symbol is
only referenced through weak references, then it becomes a <code class="code">weak</code>
undefined symbol.  If it is directly referenced, however, then such
strong references prevail, and a definition is required for the
symbol, not necessarily in the same translation unit.
</p>
<p>The effect is equivalent to moving all references to the alias to a
separate translation unit, renaming the alias to the aliased symbol,
declaring it as weak, compiling the two separate translation units and
performing a link with relocatable output (i.e. <code class="code">ld -r</code>) on them.
</p>
<p>A declaration to which <code class="code">weakref</code> is attached and that is associated
with a named <code class="code">target</code> must be <code class="code">static</code>.
</p>
</dd>
<dt><a id="index-zero_005fcall_005fused_005fregs-function-attribute"></a><span><code class="code">zero_call_used_regs (&quot;<var class="var">choice</var>&quot;)</code><a class="copiable-link" href="#index-zero_005fcall_005fused_005fregs-function-attribute"> &para;</a></span></dt>
<dd>
<p>The <code class="code">zero_call_used_regs</code> attribute causes the compiler to zero
a subset of all call-used registers<a class="footnote" id="DOCF7" href="#FOOT7"><sup>7</sup></a> at function return.
This is used to increase program security by either mitigating
Return-Oriented Programming (ROP) attacks or preventing information leakage
through registers.
</p>
<p>In order to satisfy users with different security needs and control the
run-time overhead at the same time, the <var class="var">choice</var> parameter provides a
flexible way to choose the subset of the call-used registers to be zeroed.
The three basic values of <var class="var">choice</var> are:
</p>
<ul class="itemize mark-bullet">
<li>&lsquo;<samp class="samp">skip</samp>&rsquo; doesn&rsquo;t zero any call-used registers.

</li><li>&lsquo;<samp class="samp">used</samp>&rsquo; only zeros call-used registers that are used in the function.
A &ldquo;used&rdquo; register is one whose content has been set or referenced in
the function.

</li><li>&lsquo;<samp class="samp">all</samp>&rsquo; zeros all call-used registers.
</li></ul>

<p>In addition to these three basic choices, it is possible to modify
&lsquo;<samp class="samp">used</samp>&rsquo; or &lsquo;<samp class="samp">all</samp>&rsquo; as follows:
</p>
<ul class="itemize mark-bullet">
<li>Adding &lsquo;<samp class="samp">-gpr</samp>&rsquo; restricts the zeroing to general-purpose registers.

</li><li>Adding &lsquo;<samp class="samp">-arg</samp>&rsquo; restricts the zeroing to registers that can sometimes
be used to pass function arguments.  This includes all argument registers
defined by the platform&rsquo;s calling conversion, regardless of whether the
function uses those registers for function arguments or not.
</li></ul>

<p>The modifiers can be used individually or together.  If they are used
together, they must appear in the order above.
</p>
<p>The full list of <var class="var">choice</var>s is therefore:
</p>
<dl class="table">
<dt><code class="code">skip</code></dt>
<dd><p>doesn&rsquo;t zero any call-used register.
</p>
</dd>
<dt><code class="code">used</code></dt>
<dd><p>only zeros call-used registers that are used in the function.
</p>
</dd>
<dt><code class="code">used-gpr</code></dt>
<dd><p>only zeros call-used general purpose registers that are used in the function.
</p>
</dd>
<dt><code class="code">used-arg</code></dt>
<dd><p>only zeros call-used registers that are used in the function and pass arguments.
</p>
</dd>
<dt><code class="code">used-gpr-arg</code></dt>
<dd><p>only zeros call-used general purpose registers that are used in the function
and pass arguments.
</p>
</dd>
<dt><code class="code">all</code></dt>
<dd><p>zeros all call-used registers.
</p>
</dd>
<dt><code class="code">all-gpr</code></dt>
<dd><p>zeros all call-used general purpose registers.
</p>
</dd>
<dt><code class="code">all-arg</code></dt>
<dd><p>zeros all call-used registers that pass arguments.
</p>
</dd>
<dt><code class="code">all-gpr-arg</code></dt>
<dd><p>zeros all call-used general purpose registers that pass
arguments.
</p></dd>
</dl>

<p>Of this list, &lsquo;<samp class="samp">used-arg</samp>&rsquo;, &lsquo;<samp class="samp">used-gpr-arg</samp>&rsquo;, &lsquo;<samp class="samp">all-arg</samp>&rsquo;,
and &lsquo;<samp class="samp">all-gpr-arg</samp>&rsquo; are mainly used for ROP mitigation.
</p>
<p>The default for the attribute is controlled by <samp class="option">-fzero-call-used-regs</samp>.
</p></dd>
</dl>


</div>
<div class="footnotes-segment">
<hr>
<h4 class="footnotes-heading">Footnotes</h4>

<h5 class="footnote-body-heading"><a id="FOOT7" href="#DOCF7">(7)</a></h5>
<p>A &ldquo;call-used&rdquo; register
is a register whose contents can be changed by a function call;
therefore, a caller cannot assume that the register has the same contents
on return from the function as it had before calling the function.  Such
registers are also called &ldquo;call-clobbered&rdquo;, &ldquo;caller-saved&rdquo;, or
&ldquo;volatile&rdquo;.</p>
</div>
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